In addition to primary data to be transmitted from a transmitter to a receiver, for example, data desired by the receiver, such as a transmission error detected by the transmitter or the state of the transmitter, may need to be transmitted. The primary data to be transmitted needs to be transmitted at high speed, and such data will be hereafter referred to as “high-speed data.” On the other hand, other data does not need to be transmitted at high speed and can be transmitted at lower speed than the high-speed data, and such data will be hereafter referred to as “low-speed data.”
Consider two independent devices 1, 2 between which high-speed data is transmitted as described above. If low-speed data which is information about the state of the device 1 is transmitted from the device 1 to the device 2 in this configuration, two typical methods below are conceivable.
A first method is to provide a transmission medium L1 for transmitting data and another transmission medium L2 for transmitting information about the state of the device, as shown in FIG. 1. In this method, the medium for transmitting data and the medium for transmitting information about the state of the device are independent of each other and therefore information about the state of the device can be transmitted without blocking data transmission.
A second method is to transmit information about the state of the device through a data transmission medium L1, as shown in FIG. 2. As shown in FIG. 3, this method involves transmitting primary data to be transmitted, which is high-speed data, and information about the state of the device, which is low-speed data, in a time-sharing manner. Methods for transmitting multiple types of data using a time-sharing technique are described in, for example, Patent Documents 1, 2, 3.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2009-016925    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-127799    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2000-124929